This invention relates to a method and apparatus for measuring material properties of multiple layers of films (e.g., the thicknesses of thin films) contained in a multi-layer sample.
Thin films of dielectric (e.g., polymer, oxide) and conducting (e.g., metal) materials are used in a range of microelectronic, optical, and biomedical devices. A microprocessor, for example, contains multiple layers of metal and oxide thin films deposited on a semiconducting material. Thickness variations in these films can modify the films"" electrical and mechanical properties, thereby affecting the performance of the microprocessor. Accordingly, film thickness is often monitored as a quality-control parameter during and/or after the microprocessor""s fabrication.
Several film measurement techniques, such as optical ellipsometry, reflectometry, and Impulsive Stimulated Thermal Scattering (ISTS), have been previously developed to measure oxide and metal films during fabrication of a microprocessor.
Ellipsometry is a well-known optical technique that measures the change in polarization of a light beam after it has irradiated a film. It is typically used to measure the thickness and complex refractive index of transparent (e.g., oxide, nitride) films disposed on the surface of a silicon wafer. Reflectometry is a similar optical technique that measures the change in intensity of a light beam after it has reflected off the surface of a film. Similar to ellipsometry, reflectometry measures the thickness of transparent films disposed on the surface of a silicon wafer.
ISTS is an optical technique that measures the thickness of thin metal films. It is described, for example, in pending and issued U.S. Pat. No. 5,633,711 (entitled MEASUREMENT OF MATERIAL PROPERTIES WITH OPTICALLY INDUCED PHONONS); U.S. Pat. No. 5,546,811 (entitled OPTICAL MEASUREMENT OF STRESS IN THIN FILM SAMPLES); and U.S. Pat. No. 5,812,261 (entitled METHOD AND DEVICE FOR MEASURING THE THICKNESS OF OPAQUE AND TRANSPARENT FILMS), the contents of which are incorporated herein by reference. In ISTS, a first laser pulse initiates a sound wave that propagates in a plane of the film. A second laser pulse measures a frequency of the sound wave. The frequency of the sound wave relates to film thickness.
A typical ISTS measurement determines the thickness of a single film in a multilayer stack of thin films. There exists a need, however, to measure the thickness of multiple layers in the stack of films.
In response to industry demand, the invention provides a method and apparatus for simultaneously measuring multiple properties (e.g., multiple thicknesses) of a sample composed of multiple layers of films.
In general, in one aspect, the invention provides a method for measuring properties of a sample with multiple layers of films. The method involves three basic steps: 1) irradiating a surface of the sample with at least one source of excitation radiation to excite an acoustic mode within the sample; 2) detecting the acoustic mode with a probe source of radiation which generates a signal beam that has both an oscillating component and a second, time-dependent component; and 3) analyzing a frequency of the oscillating component and analyzing at least an amplitude of the second time-dependent component, a depth of modulation of the oscillating component, or a decay constant of the second time-dependent component of the signal beam to determine at least two properties of the sample.
In one embodiment, the analysis step determines at least one property of the sample (e.g., the thickness of one layer) from the frequency of the oscillating component. At least one additional property of the sample (e.g., the thickness of a second layer) is determined from either the amplitude of the second time-dependent component, the depth of modulation of the oscillation component, or the decay constant of the second time-dependent component of the signal beam. For example, in one embodiment, one of these components is compared to a mathematical model to determine a thickness value. The mathematical model contains either an exponential component or a sinusoidal component that is used to model at least one property of the sample. This property can be the thickness of an outer film in a multilayer film structure.
In another aspect, the invention provides an apparatus for measuring a sample that includes: 1) at least one source of excitation radiation that initiates a response in a sample; 2) at least one source of probe radiation that irradiates the response to generate a signal beam that has an oscillation component and a second, time-dependent component; 3) a detector for detecting the signal beam to generate a light-induced electrical signal; and 4) an analyzer that analyzes the light-induced electrical signal to determine at least two properties of the sample from the frequency of the oscillation component and from at least the amplitude of the second time-dependent component, the depth of modulation of the oscillation component, or the decay constant of the second time-dependent component of the signal beam.
In another aspect, the invention provides an alternative method for measuring properties of a multilayer sample. This method includes: 1) irradiating a surface of the sample with a first source of excitation radiation to excite an acoustic mode within the sample; 2) detecting the acoustic mode with a probe source of radiation to generate a first signal beam; 3) irradiating a surface of the sample with a second source of radiation to generate a second signal beam; and 4) analyzing the first and second signal beams to determine at least two properties of the sample.
In one embodiment, the second probe source of radiation reflects off of the sample to generate the second signal beam. In further embodiments, the second signal beam differs from the second probe source of radiation in intensity, polarization, or a combination of intensity and polarization. This change in intensity, polarization, or combination of intensity and polarization is compared to a mathematical model to determine at least one property (e.g., thickness) of the outermost layer of the sample. The analyzer (e.g., a computer or microprocessor) determines a change in intensity between the second source of excitation radiation and the reflected second signal beam. In further embodiments, the analyzer compares the change in intensity between the second source of excitation radiation and the reflected second signal beam to a mathematical model to determine a property (i.e., thickness) of the outermost layer of the sample.
In another embodiment, the analyzer measures a change in polarization between the second source of excitation radiation and the reflected second signal beam. The analyzer compares this change in polarization to a mathematical model to determine a property (i.e., thickness) of the outermost layer of the sample.
The invention has many advantages. In general, the invention involves an all-optical, non-contact measurement technique that simultaneously and effectively measures both the thickness of thin surface films and properties (i.e., thickness, density, etc.) of underlying films, all of which are contained in a multi-layer structure. The thickness and property values can then be used to control a fabrication process (e.g., fabrication of a microelectronic device).
In the primary embodiment, unexpected functional advantages arise from determining multiple film thickness values from both the acoustic frequency and at least one of the amplitude, depth of modulation, or decay constant of the signal beam. Since each of these properties is derived from the same signal beam, both surface films and underlying layers are measured quickly and simultaneously with a single apparatus.
The apparatus features all the advantages of optical metrology: each measurement is non-contact, remote (the optical system can be as far as 10 cm or more from the sample), and can be made over a small region (as small as about 20 microns). Other properties besides film thickness may also be measured more precisely through the use of the invention.
Other features, aspects, and advantages of the invention follow from the following detailed description, and from the claims.